Various approaches have been made to provide rubber reinforced monovinyl aromatic polymers having good resistance to environmental stress cracking (i.e., good ESCR). These include the use of multi-layer sheet technology, increasing the amount of rubber, increase gel phase volume, optimizing the rubber particle size, controlling the amount of cross linking of the rubber, optimizing the process, the use of additives such as polypropylene, polybutylene, and ethylene/α-olefin copolymers, and the use of high molecular weight rubber. Some of these approaches and related technologies are described for example in US Patent Application Publication Nos. US2011/0166295 A1 (published on Jul. 7, 2011), US2010/0197863 A1 (published on Aug. 5, 2010), and US2011/0218292 A1 (published on Sep. 8, 2011); and U.S. Pat. No. 6,350,813 B1 (issued on Feb. 26, 2002), U.S. Pat. No. 4,144,204 (issued on Mar. 13, 1979), and U.S. Pat. No. 6,353,066 B1 (issued on Mar. 5, 2002), all incorporated herein by reference in their entireties. However, these approaches have had varying effects on other performance properties. For example, many of these approaches require generally a high concentration of impact modifier (rubber). In particular, when using low concentrations of rubber and high concentration of monovinyl aromatic polymer matrix (e.g., in a reactor blend where one or more monovinyl aromatic monomers are polymerized in the presence of rubber), there is generally a need to make low molecular weight monovinyl aromatic polymer matrix in order to obtain sufficiently large rubber particles for providing good ESCR.
Because of the low modulus of many compositions having good resistance to environmental stress cracking, rigid parts made of these materials are required to have generally thick walls. Thus, there continues to be a need for polymeric composition having a combination of good ESCR and high modulus, so that parts in existing applications can be down gauged and/or so that the material can be used in new applications for rubber reinforced monovinyl aromatic polymer compositions. For example, such improved balance of ESCR and modulus may allow for down gauging of liners used in refrigerators and freezers. There is also a need for such improved materials having one or more of (e.g., all of) the following characteristics: high heat distortion temperature, good impact strength, good processability, low cost, high Vicat softening point, good tensile properties, good tensile modulus, good flexural modulus, and the like.